My personal experience with this:
Yes, I have known a horse like this. She was an absolute wonder under saddle. Obedient, bold, forgiving, honest, and just a lovely ride. On the ground, she would not tolerate any form of correction. You could not correct another horse near her (even a growl, “HEY!” or “Psst!!”) without her taking offense - and her form of taking offense was coming at you with full intent to get a bite out of you (or spinning to double barrel you).
Her owner did a full workover at the equine clinic. Hormone levels, pain, tumors, ulcers, and more - there was nothing that anyone could find wrong with her.
The behaviour predated the owner I knew her with, and she came from a place that, if not outright abusive, was apparently known for “rough” handling so maybe she was always like this and that handling exacerbated it, or that handling created it - I couldn’t say. But I knew her with her last owner (who kept her until she passed of age related health issues). It never got better. She was never a safe horse for the average person to handle, and even for the capable and well equipped, one slip could cause major risk. It was also a risk for farriers and vets. The owner, or the owner’s instructor, always had to be present for all things (and the instructor at the facility was the one who would turn the mare out & bring her in, after an incident with one of the staff - who was no novice - getting kicked really nastily).
OP - don’t sign up for this circus. This is not your horse, you have no skin in this game. However nice/fancy/pretty the horse is, it sounds like the owner is doing just fine by him - he’s fed, and is managed as the weather/environment dictates, and wants for nothing.
I was asked to check on the mare I talked about, above. She was coming out of a mild colic episode and was loose in the arena, the instructor wanted me to get eyes on her and see if I thought she was doing better. Went into the arena, the mare cocked her hip at me in a way I don’t really accept (don’t come at me with your butt - ever) and I did the hiss/psst! at her.}
I can tell you I have never moved so fast in my life. I had to scramble into the center of the arena (where standards/jumps/blocks were piled) to get away, and she kept coming with full intent to kick. She only stopped when I whizzed a plastic block at her bum which startled her enough to push her away.
Don’t be me. I did not get hurt, but it was pure luck. I knew this horse and I was not rough with her, it was just a verbal cue, and that was enough to get her going.
After that, I refused to do anything with the mare and told the instructor that she had no business asking anyone else to do anything with the horse either. She was just too unsafe.